Page 48 - Koi Net - On-Line Koi Magazine - Issue 9

VEGETABLE FILTERS

Syd Mitchell explains the advantages of
using a vegetable filter.

Koi keepers often refer to ‘the nitrogen cycle’ but in
fact, in most Koi ponds, the process isn’t really a cycle
at all. Ammonia, nitrite and nitrate are all different
compounds that contain nitrogen. In the full nitrogen
cycle, the nitrogen in ammonia excreted by fish (mainly
through the gills) is converted by bugs in the bio-filter,
first into nitrite and then into nitrate. That is where we
normally leave it with the nitrate slowly accumulating in
our ponds and with only water changes or filters having
special denitrifying characteristics to reduce the level.
But, in natural waterways, the process continues with
the nitrogen in nitrate then being used as food by plants
which, in turn, are eaten by fish resulting in them
excreting more ammonia so the cycle is completed and
repeats endlessly. A Koi pond with a vegetable filter is
far nearer to the natural nitrogen cycle, especially if
watercress is grown in it, because watercress can be
fed to Koi as a source of vegetable protein and minerals
that may not be in their normal food.
Plants prefer ammonia as an energy source. This is
because ammonia contains just as much of the nitrogen
that they need as does nitrate but also, because
ammonia is such a small molecule in comparison with
nitrate, it is more easily assimilated by plant roots. In
addition, taking up nitrate actually costs plants energy
because, unlike ammonia, nitrate has an electrical
charge that has to be overcome first. The energy
required is so substantial for the plant that nitrate is only
Plants thrive on the water
chute return to the pond. taken up in daylight when energy can be obtained
during photosynthesis, whereas ammonia is so easily
absorbed by plant roots that it can be taken up even
in the dark. In addition to ammonia and nitrate, plants
in a vegetable filter also take phosphate out of the
pond water.
Left: A simple vegetable filter consists of a bed of coarse
A rectangular brick-built
planted pool provides a gravel with water flowing into it at one end to form a
base for the 4-tier shower.
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